WASHINGTON -- The Pentagon is lifting its ban on women serving in combat, opening hundreds of thousands of front-line positions and potentially elite commando jobs after generations of limits on their service, defense officials said Wednesday.

The changes, to be announced today by Defense Secretary Leon Panetta, will not happen overnight. The services must develop plans for allowing women to seek the combat positions, a senior military official said. Some jobs may open as soon as this year, while assessments for others -- special operations forces, including Navy SEALS and the Army's Delta Force -- may take longer. The services will have until January 2016 to make a case that some positions should remain closed to women.

The groundbreaking move recommended by the Joint Chiefs of Staff overturns a 1994 rule prohibiting women from being assigned to smaller ground combat units.

There has been opposition to putting women in combat, based on questions about whether they have the necessary strength and stamina for certain jobs, or whether their presence might hurt unit cohesion.

Panetta's move comes in his final weeks as Pentagon chief and just days after President Barack Obama's inaugural speech in which he spoke passionately about equal rights. The order expands the department's action of nearly a year ago to open about 14,500 combat positions to women; nearly all in the Army. Panetta's decision could open more than 230,000 jobs -- many in Army and Marine infantry units -- to women.

There also have been suggestions that the American public would not tolerate large numbers of women being killed in war.

Under the 1994 Pentagon policy, women were prohibited from being assigned to ground combat units below the brigade level. A brigade is roughly 3,500 troops split into several battalions of about 800 soldiers each. Historically, brigades have been based farther from front lines and often included top command and support staff.

The necessities of combat in Iraq and Afghanistan propelled women into jobs as medics, military police and intelligence officers who sometimes were attached -- but not formally assigned -- to battalions. While a woman couldn't be assigned as an infantryman in a battalion going out on patrol, she could fly a helicopter supporting the unit, or provide medical aid if troops were injured.

The Joint Chiefs of Staff unanimously agreed to send the recommendation to Panetta earlier this month.

A senior military official familiar with the discussions said the chiefs concluded this was an opportunity to maximize women's service. The official said the chiefs of the Army, Navy, Air Force and Marine Corps laid out three main principles to guide them as they move through the process:

* They were obligated to maintain America's effective fighting force.

* They would set up a process that would give all service members, men and women alike, the best chance to succeed.

* They would preserve military readiness.

Recent surveys and experiences have shown, it will not be an easy transition. When the Marine Corps sought women to go through its tough infantry course last year, two volunteered and failed to complete the course. There may not be a wide clamoring from women for the more intense, dangerous and difficult jobs -- including some infantry and commando positions.

In the Navy, women have begun moving into the submarine force, with several officers already beginning to serve.

Jon Soltz, who served two Army tours in Iraq and is the chairman of the veterans group VoteVets.org, said it may be difficult for the military services to carve out exceptions to the new rule. While he acknowledged that not all women are interested in pursuing some of the gritty combat jobs, "some of them are, and when you're looking for the best of the best you cast a wide net. There are women who can meet these standards, and they have a right to compete."

Two lawsuits were filed last year challenging the Pentagon's ban on women serving in combat, adding pressure on officials to overturn the policy. And the military services have been studying the issue and surveying their forces to determine how it may affect performance and morale.

Part of the process, one official said, would allow time to get female service members into leadership and officer positions in some of the more difficult job classifications to help pave the way for female enlisted troops.

"Not every woman makes a good soldier, but not every man makes a good soldier. So women will compete," said Rep. Loretta Sanchez, D-Calif. "We're not asking that standards be lowered. We're saying that if they can be effective and they can be a good soldier or a good Marine in that particular operation, then give them a shot."

Women make up about 14 percent of the 1.4 million active military personnel. More than 280,000 women have been sent to Iraq, Afghanistan or to jobs in neighboring nations in support of the wars. Of the more than 6,600 who have been killed, 152 have been women.

The senior military official said the military chiefs must report to Panetta with their initial implementation plans by May 15.

If a military draft ever were to be reinstated, changing the rules would be difficult. The Supreme Court has ruled that because the Selective Service Act is aimed at creating a list of men who could be drafted for combat, American women are not required to register upon turning 18 as are all males.

If combat jobs open to women, Congress will have to decide what to do about that law.